Microsurgical Wiltse Paraspinal Approach Using a Low-Budget Exoscope

Access to microneurosurgical care in low- and middle-income countries remains limited mainly due to a lack of equipment. High purchasing and maintenance costs hinder the use of operating microscopes in low-resource facilities. The authors present an improved version of their previously introduced low-cost exoscope to achieve high magnification and illumination in low-resource environments. The setup included a 48-megapixel two-dimensional digital microscope camera, a wide field C-mount lens, ring light, and a two-link cantilever with a screw terminal. The surgical field was projected to a portable 17.3-inch 2K resolution monitor. Ten patients underwent exoscope-assisted transforaminal lumbar interbody fusion via the Wiltse paraspinal approach. The simple construction allowed a fast and intuitive preoperative setup. The in-plane switching type display provided a clear and bright image regardless of the viewing angle. The two-link arm of the cantilever allowed smooth positioning of the camera, overcoming the cumbersome up and down movements needed to zoom in and out with the previous prototype. Industrial microscope cameras are effective low-budget alternatives to conventional operating microscopes in lumbar microdiscectomy. The improved system is superior compared to the authors' previous prototype with regard to affordability, image quality, and adjustability of position and angle.


Introduction
Only a quarter of the world's population has access to microneurosurgical facilities within a range of two hours. High acquisition and maintenance costs hinder the use of operating microscopes and exoscopes in low-resource environments [1]. The authors recently described a simplified exoscope composed of industrial digital microscope parts. They reported sufficient magnification and illumination in 13 spinal and three cranial surgeries. Yet, their setup showed considerable limitations including the cumbersome manual adjustment of zoom and camera position [2]. Here, the authors introduce a new version providing smoother adjustability at only half the cost of the initial prototype. The new system was tested in a cohort of 10 patients undergoing the microsurgical Wiltse paraspinal approach for transforaminal lumbar interbody fusion (TLIF) surgery.

Device properties
The authors purchased a 48 megapixels light-weight video microscope camera and a wide-field industrial Cmount lens (Eakins, Shenzhen Huaxin Electronic Trading Limited Company (Co., Ltd.), Shenzhen, China). An LED ring light provided shadow-free focused illumination and adjustable brightness. A cantilever with a two-link arm allowed for manual adjustment of the camera position and angle with a wide range of motion (Eakins, Shenzhen Huaxin Electronic Trading Co., Ltd., Shenzhen, China). The cantilever was attached to the operating table through a screw terminal. The surgical field was projected to a portable 2K high-resolution in-plane switching (IPS) monitor placed at eye level at a 100-to 120-cm distance (Porpoise, China). The total cost of the present setup was roughly US$ 350 ( Table 1). The authors' previously published setup was purchased at a total cost of US$ 750 ( Figures 1A, 1B) [2].   The cantilever, LED ring light, and C-mount lens were sterilized with ethylene oxide. The microscope camera and cables were covered with sterile drapes (Figures 2A, 2B).

Patient characteristics
The exoscope was tested in six female and four male patients. The mean age was 52.2 (39-70) years. One patient underwent a TLIF at the L3-L4 segments, six at the L4-L5 segments, and three at the L5-S1 segments. All patients presented with lower back pain and radiculopathy with mild to moderate sensorimotor deficits. Individual patient consent was obtained prior to enrollment.

Surgical technique
The Wiltse approach is an old technique for achieving a minimally invasive corridor to the posterolateral spine. Minimal intraoperative bleeding, short hospital stay, and low infection rates render the Wiltse approach an elegant alternative to the standard midline posterior approach [3]. Surgery was performed under general anesthesia. Patients were operated on in prone positions. Intraoperative C-arm fluoroscopy helped exactly locate the affected segments. Two paravertebral incisions were made 4 cm lateral to the midline. The thoracolumbar fascia was opened longitudinally roughly 3 cm lateral to the spinous processes using monopolar cauterization. The medial multifidus muscle was mobilized away from the lateral longissimus muscle to open a natural corridor to the junction between the facets and the transverse process ( Figures 3A, 3B).

FIGURE 3: (A) The Wiltse approach begins with a paravertebral incision 4 cm lateral to the midline. Using blunt dissection, the natural corridor between the multifidus and longissimus muscles is opened. (B) The junction between the facets and the transverse process is exposed.
A Caspar retractor system for minimally invasive spine surgery was installed and a Penfield dissector no. 4 was inserted for radiological confirmation of the correct segment. Pedicle screws were inserted on both sides ( Figures 4A, 4B). A tubular retractor with a 22 mm diameter was inserted.

FIGURE 4: (A) Sagittal and (B) coronal x-ray images of the lumbosacral spine after bilateral pedicle screw placement at the L4/L5 segments in a 44-year-old male patient.
The authors proceeded under microscopic vision using the low-cost exoscope (Video 1). The cantilever was mounted to the contralateral side of the operating table. A laminectomy and facetectomy were performed by removing the inferior articular process and the superior third of the superior articular process.
VIDEO 1: High-resolution footage of the surgical field as captured by the low-cost exoscope, following removal of the right inferior facet in a 44-year-old male patient.
View video here: https://www.youtube.com/watch?v=0WMwZahpUu8 A flavectomy was performed to expose the dural sac and intervertebral disc. Following microsurgical discectomy, an interbody cage filled with bone autograft was placed and screws were connected through rods. The exoscope-assisted stage of the surgery accounted for approximately a quarter of the total operation duration ( Table 2). Pain relief was achieved in all patients. Nine showed full sensorimotor recovery, one showed partial motor recovery at a six-week follow-up. There were no intraoperative complications and the mean blood loss was 198 (110-260) mL.

Patient
No.

User experience
The simple construction of the exoscope allowed intuitive preoperative preparation in under five minutes. The screw terminal at the base of the cantilever enabled fast installment to the operating table when proceeding to the microsurgical stage of the surgery. Alternatively, the cantilever could be attached to an intravenous pole like the previous prototype. The surgical field was projected to a portable 17.3" 2K highresolution monitor. Albeit small, the IPS-type panel provided a clear and bright image regardless of the viewing angle. This allowed the assistant surgeon and the nurse to follow the surgery on the same screen ( Figure 5). This was particularly beneficial for teaching younger staff. Operative footage could be recorded although our 16 GB micro SD card provided storage space for a maximum of two surgeries only. Video files were thus transferred to a hard disk following each surgery. The lack of three-dimensional (3D) vision remained a main limitation in this new prototype, too. The ring light achieved sufficient illumination with no need for additional overhead lamps. Lighting was best when the camera was set at a working distance of 40 to 60 cm. The authors did not experience any significant glare issues. The two-link arm of the cantilever allowed smooth positioning of the camera, overcoming the cumbersome up and down movements needed to zoom in and out with the previous prototype. The small size of the setup allowed switching between microscopic and macroscopic views without having to move the device away from the surgical field. The overall user experience was superior compared to the initial system.